This invention relates to a sealed type lead acid storage battery, in particular, a battery in which oxygen gas generated during operation or charging is absorbed in the negative electrode plate. In a conventional battery of this kind, a cell unit comprises a separator of 0.5-3.0 mm in thickness comprising a single layer which consists only of glass fibers having an average diameter of 0.1-3.0.mu., or a single layer which consists mostly of the foregoing glass fibers mixed with a small amount of fibrillation synthetic fibers. The separator is interposed between a negative electrode plate and a positive electrode plate to form a cell unit. The desired number of cell units are compressed in a battery container and an aqueous dilute sulfuric acid electrolyte is impregnated in and held by the separators.
The above conventional battery is, however, disadvantageous in that although the separator has a density as low as 0.2 g/cm.sup.3 and is a good absorbent of the electrolyte, when the separator is impregnated with the electrolyte its thickness is decreased. Also, as the amount of impregnated electrolyte becomes smaller, the thickness of the separator is liable to decrease. Consequently, the initial good close contact of the separator with the opposing surfaces of the negative electrode plate and the positive electrode plate is lost. As a result, the performance of the battery and its residual capacity are readily reduced, shortening the service life of the battery or causing other problems.
In order to minimize the above disadvantages, the decrease in the thickness of the separator caused by the impregnation of the electrolyte in the battery cells is usually compensated as much as posible by increasing the compression force applied to the assembly of cells contained in the battery container. However, the density of the separator is consequently increased, therefore reducing its absorption of the electrolyte. As a result, the amount of the electrolyte impregnated in the separator is liable to become insufficient, and the performance of the battery is significantly poor.
Accordingly, it is desirable to provide a battery wherein the separators retain good electrolyte-absorbing and holding properties for a long time, and do not exhibit a decrease in thickness as a result of being impregnated with the electrolyte.